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EARTH SURFACE PROCESSES AND LANDFORMSEarth Surf. Process. Landforms 34, 1255–1269 (2009)Copyright © 2009 John Wiley & Sons, Ltd.Published online 22 May 2009 in Wiley InterScience(www.interscience.wiley.com) DOI: 10.1002/esp.1813

John Wiley & Sons, Ltd.Chichester, UKESPEarth Surface Processes and LandformsEARTH SURFACE PROCESSES AND LANDFORMSEarth Surface Processes and LandformsThe Journal of the British Geomorphological Research GroupEarth Surf. Process. Landforms0197-93371096-9837Copyright © 2006 John Wiley & Sons, Ltd.John Wiley & Sons, Ltd.2006Earth ScienceEarth Science99999999ESP1813Research ArticleResearch ArticlesCopyright © 2006 John Wiley & Sons, Ltd.John Wiley & Sons, Ltd.2006

Sediment transport due to tree root throw: integrating tree population dynamics, wildfire and geomorphic responseSediment transport due to tree root throw

J. M. Gallaway,1,2 Y. E. Martin1,2* and E. A. Johnson2,3

1 Department of Geography, University of Calgary, Calgary, Alberta, Canada 2 Biogeoscience Institute, University of Calgary, Calgary, Alberta, Canada 3 Department of Biological Sciences, University of Calgary, Calgary, Alberta, Canada

Received 23 September 2008; Revised 16 February 2009; Accepted 23 February 2009

* Correspondence to: Y. E. Martin, Department of Geography, University of Calgary, Calgary, AB T2N 1N4, Canada. E-mail: [email protected]

ABSTRACT: A field study was conducted to analyze root throw and associated sediment transport in Hawk Creek Watershed,Canadian Rockies. A large crown fire in 2003 allowed the opportunity to study pre-fire and post-fire root throw. Based on fielddata, a significant relation was found between gradient and root plate volume, as well as individual root plate dimensions. Giventhat tree diameters increase as trees age and that a relation in the field data was found between tree diameter and root platevolumes, sediment transport due to root throw is expected to change in response to forest disturbance and stand age. Sedimentdisturbance, which is the amount of sediment upheaved during tree topple and does not take into account transport distance,shows higher values on steeper gradients. Sediment transport was notable for the steepest plots, with pre-fire values of0·016 cm3 cm–1 a–1 and post-fire values of 0·18 cm3 cm–1 a–1. A tree population dynamics model is then integrated with a rootthrow transport model calibrated for the Canadian Rockies to examine the temporal dynamics of sediment transport. Fire isincorporated as a disturbance that initiates development of a new forest, with the model cycling through generations of forest.Trees fall according to an exponential rate that is based on time since death, resulting in a time lag between tree mortality andsediment transport. When values of time-since-previous-fire are short, trees are generally <13 cm, and minimal sediment isupheaved during toppling. If trees reach a critical diameter at breast height (dbh) at time of fire, a pulse of sediment occurs in theimmediate post-fire years due to falling of killed trees, with tree fall rates decreasing exponentially with time-since-fire. A secondpulse of root throw begins at about 50 years after the previous fire, once new recruits reach a critical dbh and with initiation ofcompetition-induced mortality. Copyright © 2009 John Wiley & Sons, Ltd.

KEYWORDS: tree root throw; sediment transport; wildfire; tree topple

Introduction

Tree topple, which may involve stem breakage or uprooting,plays a critical role in forest population dynamics (for reviewsee Quine and Gardiner, 2007), with events ranging in sizefrom severe storm tree blow downs of kilometers in length toindividual tree death due to competition, insects, or diseaseand subsequent tree topple due to decay of the supportingroot structures or the stem. Root throw is defined as treeuprooting when the root plate is upheaved along with anyattached sediment. Root throw is recognized as an importantnear-surface process affecting infiltration, air capacity, andremixing of organic material (e.g. Lutz, 1940; Meyers andMcSweeney, 1995; Clinton and Baker, 2000) and is also animportant sediment transporting agent on forested hillslopes(e.g. Dietrich et al., 1982; Swanson et al., 1982; Roeringet al., 2002; Gabet et al., 2003; Osterkamp et al., 2006).Root throw results in vertical and horizontal displacementof sediment attached to the roots (called the root plate). Thedisturbed sediment often remains attached to the root plate

for a period of time after root throw. Subsequent root platedisintegration due to weathering and decay of the roots leadsto vertical fall of sediment, which may remain in situ or movehorizontally and/or vertically due to gravity and inertia. A pit-mound pair is the resulting geomorphological feature fromthis process (Stephens, 1956). Mound degeneration mayoccur through weathering and transport processes, such asrainsplash or diffusive creep. Subsequent mound disintegrationis not considered as part of the root throw process herein.

The volume of soil disturbed during a root throw eventdepends on factors including: tree species and age; whetherthe tree was alive or dead at time of fall; soil texture; rootingstructure and depth; and moisture content of soil at time ofthe event (Norman et al., 1995). Root plate volumes are smallerfor trees that have been dead for some time prior to falling(Cremeans and Kalisz, 1988; Norman et al., 1995; Ulanova,2000), due to decay of fine roots and reduced cohesion betweenthe root structure and the soil (Swanson et al., 1982). Thenature of how the tree topples determines the location of theroot plate relative to the pit, which in turn affects the fall

Copyright © 2009 John Wiley & Sons, Ltd. Earth Surf. Process. Landforms 34, 1255–1269 (2009)DOI: 10.1002/esp

1256 EARTH SURFACE PROCESSES AND LANDFORMS

location of disintegrating sediment (e.g. Beatty and Stone,1986). The root plate may be situated either upslope or down-slope of the originating pit (with some lateral component alsopossible), or may be situated directly above the pit. Hillslopegradient, angle of tree fall relative to contour lines, and finalresting location of the root plate are key factors affecting thevolume of sediment that ultimately falls either outside orwithin the pit (Norman et al., 1995). Sediment that falls fromthe root plate may contribute to downslope sediment transport,but in some cases sediment may be deposited upslope of thepit, or it may be deposited in the pit itself. Finally, the rate ofsediment disintegration affects the final transport rate (assumingthe transport process is only completed once sediment depositson the ground surface), with the disintegration rate beinginfluenced by factors such as root plate dimension, sedimentparticle size, and weathering intensity.

How important is root throw relative to other hillslope geo-morphic processes? Gabet et al. (2003) estimated sedimenttransport rates for root throw using a model calibrated withfield data, and obtained values of the order 10–3 m3 m–1 a–1.Roering et al. (2002) estimated sediment transport rates dueto biogenic processes (e.g. root growth, root throw) of the order10–2 to 10–3 m3 m–1 a–1. For comparison, these values are closeto or somewhat higher than typical soil creep rates reportedin other studies [see Martin (2000) for compilation of publishedcreep rates], and lower than mass wasting rates due to shallowlandsliding in coastal British Columbia (Martin, 2000; Martinet al., 2002).

To the best of our knowledge no published studies haveexplicitly connected the timing and rates of sediment transportdue to root throw to tree population dynamics driven by wildfiredisturbance. Furthermore, root throw studies have not beenundertaken in the forests of the Canadian Rocky Mountains.The primary objective of this study is to develop a model thatintegrates tree population dynamics (i.e. tree recruitment,growth, mortality, toppling) with sediment transport due toroot throw for this region. To achieve this goal, a field programwas undertaken to document root throw occurrences andcharacteristics, and associated sediment transport, for bothpre-fire and post-fire scenarios in the Canadian Rockies. Inparticular, a knowledge of root plate characteristics in theseforests based on field evidence is essential to the calibration

of our integrated forest population dynamics/sediment transportmodel. To study the temporal dynamics of root throw overscales of 103 years, the model cycles through generations offorests and associated root throw as conditioned by crownfire disturbances which cause death of the tree population. Usingthis integrated model, we strive for improved understandingof how tree population dynamics driven by wildfire disturbanceinfluences the temporal dynamics of sediment transport dueto root throw in the Canadian Rockies.

Study Area

The field program was undertaken in Hawk Creek, KootenayNational Park, south-eastern British Columbia (Figure 1). Theregion is underlain by folded and faulted sedimentary rocks,which were uplifted during the Cretaceous-Tertiary with eleva-tions ranging from 800 m to 3400 m. The major valleys havebeen glaciated, resulting in u-shaped main valley floors andhanging valleys for many tributaries. Hawk Creek drainagebasin is approximately 24 km2 in area and is a fourth ordertributary of the major Vermillion River. Elevation is 1330 mat the confluence with the Vermillion River, and 3086 m inthe upper slopes. Hillslope gradients are moderate (generally<30º) in the lower third of the basin, where the study plotsare situated, and become steeper (up to 45°–50°) with increas-ing elevation. Slopes immediately adjacent to Hawk Creekhave steep gradients (25°–40°) for much of the main stemlength and often constrain development of a floodplain andriparian zone. The lower portion of the basin contains morainalmaterial overlying bedrock and includes bedrock outcropsand small areas of colluvium. The soils are unconsolidatedand unsorted, with a significant number of cobbles and boulderswithin the matrix. The matrix is a silty-sand, with clay content<2% or absent.

The temperatures in Kootenay National Park are influencedby cold continental air masses from the north or maritime windsfrom the west. The continental divide at the northeast boundaryallows for greater influence by maritime weather patterns,resulting in a somewhat milder and moister climate than eastof the divide. Winter precipitation is largely in the form ofsnow, averaging about 170 cm annually; summer rainfall is

Figure 1. Hawk Creek, Kootenay National Park, British Columbia, Canada. Extent of the 2003 burn is shown in the right-hand figure.


SEDIMENT TRANSPORT DUE TO TREE ROOT THROW 1257

frequently delivered by convectional thunderstorms, with anaverage annual rainfall of about 340 mm. Precipitation increaseswith elevation throughout the region (Environment Canada,2005).

The vegetation for the Vermilion basin consists of subalpineforest and alpine tundra. The lower subalpine forest is composedprimarily of lodgepole pine (Pinus contorta Loudon var. latifoliaEngelm.) and Engelmann spruce (Picea engelmannii Parry ex.Engelm.), and the upper subalpine forest of Engelmann spruceand subalpine fir (Abies lasiocarpa [Hook.] Nutt.).

The fire season is from May to September, with peaklightning activity in July and August (Masters, 1990). Crownfires are the dominant type of fire. Masters (1990) calculateda return interval for the study area of 75 years for pre-1768data, and 267 years for post-1768 data. The change in fireinterval during the 1700s is related to the Little Ice Age, ashas been found for other Rocky Mountain locations (Johnsonand Larsen, 1991). These fire return intervals are less than thepotential lifespan of the canopy trees (250–300 years, possiblyup to 375 years), making wildfire an important determinant oftree population dynamics. The duff layer (F and H organic layersof the soil) in unburned forests of this region is continuousand up to 15 cm thick. As in other Canadian Rocky Mountainlocations, crown fires consume large amounts of the duff layer.

In late July 2003, lightning ignited two fires in the Vermil-lion Valley. These fires merged to burn approximately 17 000hectares (see Figure 1). About 80% of the forested area inHawk Creek basin was burned by high intensity (crown)wildfire, including much of the riparian zone. Only the veryupper part of the drainage basin did not burn. The previousburn date at Hawk Creek was in 1835 (Masters, 1990).

Field Methods

To estimate rates of root throw and associated sedimentvolumes, three plots, having areas of 2·5, 4·1, and 3·2 ha(Plots 1, 2, and 3, respectively), were delineated in the lowerreaches of the Hawk Creek drainage basin. The plots werelocated within 150 m of each other, had a southwest aspectand hillslope gradients of 3°, 15° and 28°, but maintainedreasonable consistency in most other physical attributes. Allfield measurements were made after the fire. The collectionof pre-fire data for root throw was based on informationderived from fallen trees and root plates, which were inferredto have been in place at the time of fire based on certainobserved characteristics. Root throw frequency and charac-teristics were monitored in the first two years following the

fire. Topples caused by a break or snap in the bole were notconsidered as no sediment upheaval is associated with thistype of tree fall event; this study focuses exclusively on rootthrow events (i.e. tree topples that upheave sediment throughan uprooting event). Root plates for fallen trees were surveyedand flagged to allow identification of new topples that occurredbetween surveys and to monitor root plate disintegration.Root plates were included in the survey if sediment was stillattached to the root structure.

Four root plate age classes, modified from Brown et al.(1998), were derived for the current study (see Table I). Basedon the characteristics listed in Table I, root plates were assignedto one of three pre-fire age classes, or to the post-fire category.In the first-year of field work (2004), post-fire topples wereidentified by unburned roots at the base of the root plate. Allroot plates, including those which existed before the fire andthose that came into existence due to post-fire toppling, wereflagged to allow identification of new topples in 2005. Post-fire root throw frequency was monitored by counts of treefalls in the three plots. Forest density defines the number oftrees available for toppling, and was estimated by tree countsof standing trees with diameter at breast height (dbh) ≥10 cmin six sample tracts (each ~150 m2) within each plot.

Surveys of existing root plates in each plot were performedin the summers of 2004 and 2005. Detailed data werecollected for root plates within Age Classes 1–3. Root platesin Age Class 4 were generally more deteriorated, reducingmeasurement accuracy for bole, root plate, and disintegrationrates; therefore, they were not included in detailed fieldmeasurements of sediment volumes or root plate disintegra-tion, but were only included in bulk volume estimates.

The detailed analysis is as follows. Fall directions for rootplate bole and angle of fall relative to local contour wererecorded. Fall angles were recorded in Survey Plots 2 and 3,where hillslope gradient was sufficient to allow identificationof direction of steepest ascent. The two areal dimensions for aroot plate are designated as width and height. Measurementsof root plate width (w) were made parallel to the groundsurface, and height (h) measurements were taken orthogonalto width. Measurements were normally taken on the undersideof the root plate (i.e. the rounded side with newly exposedsoil), but in some cases, pit infilling or pit-plate configurationforced measurements to be made on the ground surface sideof the root plate. It is important to distinguish betweenthe dimension that is parallel and that which is normal to theground surface for later sediment transport calculations. Theprocedure to determine depth (d) of the root plate requiredtwo measurements. The first measurement was made by placing

Table I. Root throw age classes based on bole description; modified from Brown et al. (1998) to simplify application in apost-fire environment

Age class

Shortdescription Characteristics

Age estimates at timeof first measurement

1 New Roots at bottom of root plate not burned so tree fell after the fire

Post-fire

2 Recent Fully barked (>80%); bole solid; no checkerboard burn pattern on bole, indicating tree was not dead long prior to burning

0–2 years before fire

3 Deteriorating Bark 0–80%; some sapwood decay but bole generally whole; checkerboard burn pattern on some or all of bole


4 Old Sapwood flaking, easily removed; settling of stem or flattening of circumference; checkerboard burn pattern on all of bole


Note: Bole description includes amount of bark on the bole, burn pattern, and amount of soft or rotting bole.



a rod at right angles to the bole where the ground surfaceintersected the trunk, and measuring from this rod to the planeestimated to pass through the outside edge of the roots. A seconddepth measurement was made from the rod to the maximumdepth of the root plate. The difference between these two meas-urements defines depth for the half ellipsoid. This approachexcludes sediment that may exist in the root plate directly beneaththe bole, which may partially compensate for unrealistic smooth-ness of the half ellipsoid surface that may overstate the volumein the lower portion of the root plate. For asymmetrical rootplates with considerable differences in depth across the plate,multiple depth measurements were taken and averaged.

The half-ellipsoid model of Denny and Goodlett (1956) andNorman et al. (1995) was used to calculate the root platevolume (Figure 2):

(1)

where VRP is volume of root plate (in m3), w is width (inmeters), h is height (in meters), and d is depth (in meters).Visual estimation was made of the percentage (0, 25%, 50%,

75%, 100%) of root plate sediment still attached to the rootstructure and the percentage (0, 25%, 50%, 75%, 100%) ofroot plate sediment that would eventually fall outside of the pit.Estimates were limited to these values to facilitate consistencyand repeatability, and were performed by multiple personnelin the field. Older root plates were only included in totalvolume estimates (i.e. volume estimates summed for all rootplates), and were classified based on width of root structure(small: <75 cm, medium: 75–150 cm, large: >150 cm).

In addition to the collection of root plate data, a meteoro-logical station recorded precipitation, wind speed, and winddirection over the study period. Additional data recorded foreach tree topple included: (i) position on hillslope within theplot (bottom third, middle third, upper third); (ii) bole dbh(taken at 1·4 m above ground); and (iii) tree species.

Field Results and Analysis

Root throw frequency and direction

The detailed survey included 166 root plates, with a further 143older root plates classified as small/medium/large. No notable

Figure 2. (A) Area dimensions associated with a root plate. View is shown looking at underside of root plate. (B) Left-hand photograph showsarea dimensions for a root plate. Right-hand photograph shows depth of a root plate. Photograph by J. Gallaway, Kootenay National Park, BritishColumbia, June 2004.

Vw h d

RP = × ×⎛⎝⎜

⎞⎠⎟

23 2 2 2π



trends are found in the frequency rate per hectare of root throwamongst the three gradient classes (Table II). When tree densityis taken into consideration, there are still no obvious patternsin the data amongst the gradient classes. The data are thensplit up and analyzed according to pre-fire and post-fire rootthrow events (Table III). Data for pre-fire events show no notableincrease in rates with average gradient, with values rangingfrom 0·29 to 0·37 events annually per hectare. The post-firedata indicate increasing root throw frequency with gradient.The disparity between pre-fire and post-fire rates increaseswith increasing gradient, showing notable differences for thesteepest gradient class of 28º.

Root throw data show a mean fall direction of 29º (Figure3A). The wind direction recorded during the study period wasmost frequently between 20° and 30°, a range that includesthe mean direction of tree fall. This wind direction acrosslocal topography is predominantly uphill, and the effect isapparent in the distribution of tree fall angles relative tocontour lines (Figure 3B). Approximately 65% of the tree fallswere uphill relative to local contour lines.

Root plates

When all root plates are considered, the relation betweendbh and root plate volume is (Figure 4):

VRP = (−5·74 × 10−4 × dbh) + (5·82 × 10−3 × dbh2) R2 = 0·45 (2)

where VRP is root plate volume (in m3) and dbh is diameter atbreast height (in cm). The mean value of root plate volumeincreases from a value of 0·2 m3 to 0·71 m3 (p < 0·01) asgradient becomes steeper (Table IV).

Width and height of the root plate were defined earlier aswidth being parallel to the ground surface and height being

Figure 3. (A) Rose diagram showing tree fall direction in degrees. (B) Histogram showing frequency of uphill and downhill topples. A value of 0ºrepresents an uphill fall direction (perpendicular to hillslope contour lines) while a value of 180º represents a downhill fall direction, with othervalues falling in between.

Table II. Root plate counts for the three survey plots

Topple survey plot

Average gradient (deg)

Plot area (ha)

Tree density(# ha–1)

Root platecount

Root plates perhectare (# ha–1)

Frequency(% ha–1)

1 3 2·5 1665 85 34·0 2·02 15 4·1 1882 112 27·3 1·53 28 3·2 2200 112 35·0 1·6

Note: Root plate counts relative to the tree density for each plot are shown in the last column.

Table III. Annual topple rates by gradient

Topple survey plot


Plot area (ha)

Pre-fire topple rate (# ha–1 yr–1)

Pre-fire topplerate (% yr–1)

Post-fire topple rate (# ha–1 yr–1)

Post-fire topplerate (% yr–1)

1 3 2·5 0·37 0·022 0·40 0·0242 15 4·1 0·29 0·015 0·60 0·0323 28 3·2 0·35 0·016 1·7 0·078

Note: Time span for pre-fire root plates is 90 years; time span for post-fire root plates is two years. Rates are presented per hectare, and as percentof standing tree density.

Figure 4. Relation between diameter at breast height (dbh) and rootplate volume. Data are shown for Age Classes 1–3 (32-year period).



normal to the ground surface. However, in that definitioneither width or height could be the larger of the two dimensions,likely meaning that empirical analysis of relations for changesin a particular dimension with increasing slope gradientwould be obscured. Therefore, for this particular analysis andtable (Table IV, columns three to five), the shorter dimensionof the two is referred to as ‘width’ and the longer dimensionas ‘length’; the depth dimension remains the same as definedearlier. Increases in magnitude with slope gradient for eachindividual dimension (width, length and depth) were found tobe significant at p < 0·01. On slopes having a gradient >0°,mechanical stress is expected to be unevenly distributedaround the tree diameter and the root system. Root growthmay respond by increasing the density of the root system orby arranging roots in an asymmetric manner in the directionof maximum stress, both of which serve to provide increasedanchorage for the tree and greater stability (Soethe et al.,2006; Coutts et al., 1999). Unfortunately, our data do notallow us to determine the direction in which our root plateswere oriented when originally in the ground. Nonetheless,it is still of interest to assess changes in the ratio of lengthto width for root plates measured in the field. A significantincrease is found for the ratio of length to width with increasingslope gradient (Table IV, column six), lending some support tothe idea that asymmetry of the root plate will become morepronounced as gradient increases.

Disintegration rates of root plates were determined byestimating percentages of volume removed from the rootplates in 2004 and 2005 (Table V) for the three age categories(1–2 years post-fire, two years pre-fire and 30 years pre-fire;refer back to Table I). As of 2004, the youngest (most recent)root plates had similar percentages of volume removed fromthe root plate as the older category. Furthermore, in 2005 itwas these youngest root plates that had lost the greatestamount of volume in the intervening period. For both pre-fireand post-fire scenarios, the steepest-gradient plots showed thegreatest rates of sediment disintegration. A greater proportionof sediment associated with root plates falls outside the

originating pit on steeper gradient slopes relative to gentlerslopes, possibly due to a greater occurrence of rotational fallson steeper slopes (Table VI). For both pre-fire and post-fireroot plates the average percent falling outside the pit increasesfrom negligible values for relatively flat land surfaces, throughto 20–30% for the mid-slope category and up to about 50%for the steepest gradients.

Areal sediment disturbance and soil turnover

The total area of ground surface disturbed by tree upheaval(i.e. total area disturbed by all events occurring during aspecified time period) was based indirectly on the root platemeasurements and covers all four age categories, or a periodof approximately 92 years (Table I). The area subject to treeroot upheaval increases with slope gradient, with annualpercentages of disturbed land surface ranging from 0·003%to 0·006% (Table VII).

Sediment disturbance and transport

Amounts of sediment disturbance include the volume of allsediment that is uprooted, whether it is eventually returnedto the pit or not. Sediment returned to the pit during rootplate disintegration contributes to weathering and breakdown

Table IV. Mean values of root plate dimensions for the three survey plots

Gradient classRoot plate volume (m3) Width (cm) Length (cm) Depth (cm) Ratio of length to width

1 (n = 45) 0·22 (0·11) 91·2 (80) 117·1 (110) 26·9 (25) 1·33 (1·24)SE = 0·044 SE = 6·0 SE = 6·8 SE = 1·9 SE = 0·043

2 (n = 61) 0·35 (0·18) 115·5 (106) 160·0 (145) 29·7 (28) 1·42 (1·30)SE = 0·058 SE = 6·9 SE = 10·7 SE = 1·9 SE = 0·0505

3 (n = 60) 0·71 (0·62) 150·4 (155) 219·5 (223) 39·3 (40) 1·49 (1·43)SE = 0·072 SE = 5·6 SE = 9·3 SE = 2·1 SE = 0·0488

Note: Median values are given in brackets, and the standard error (SE) is indicated.

Table VI. Root plate disintegration data for the three survey plots

Topple survey plot

Root plate counts

Average percentfalling outside pit

Average percent off root plate 2004

Average percent off root plate 2005

Pre-fire root plates1 43 5·0 46 542 56 30 46 533 49 52 33 53

Post-fire root plates1 2 0·0 38 382 5 20 68 803 11 50 55 73

Note: Pre-fire and post-fire root plates are analyzed separately.

Table V. Root plate disintegration averages by age class

Ageclass Age maximum

Average volumeoff 2004 (%)

Average volumeoff 2005 (%)

1 1–2 years post-fire 48·3 70·82 2 years pre-fire 33·3 47·73 30 years pre-fire 46·7 54·0

Note: Data are for detailed root plates only.



of the soil layer, while sediment that disintegrates and landsoutside the pit contributes to sediment transport. The transportdistance of sediment after uprooting becomes important inour calculations of sediment transport (see Equation 3) and isnot a consideration when estimating sediment disturbancevolumes. Sediment disturbance rates over a 32-year period(time period for root plates of Age Classes 1–3) are evaluatedto assess if there is a gradient dependency on the total amountsof uprooted sediment over this period. Results do not show anotable difference in total volume uprooted per square meterof hillslope for the two lowest-gradient study plots, with valuesof 1·25 × 10–5 and 1·63 × 10–5 m3 m–2 a–1 respectively (Table VIII).However, the steepest plot, Plot 3, does show a notably highervalue of 4·15 × 10–5 m3 m–2 a–1 compared to the other plots, inlarge part due to the higher root plate volumes associatedwith toppled trees at steeper gradients as discussed earlier.

We now consider sediment transport rates due to root throw.It is assumed that sediment falling into the pit undergoes nonet transport, and thus only sediment that is involved informing mounds is considered in this analysis. A diffusiveapproach to sediment transport may be appropriate if rootthrow is considered to be a relatively slow, quasi-continuousprocess with a dependency on hillslope gradient (Normanet al., 1995), and with the potential to operate across the entireforested portion of the landscape. Adopting such an approachallows for comparison with transport rates for other processesinvolved in medium-term drainage basin evolution (note thatthe temporal dynamics of root throw transport will be exploredin more detail in the modeling section of this paper).

The equation used to calculate the sediment transport ratefor a plot of a given gradient is (adapted from Martin andChurch, 1997):

(3)

where qs is sediment transport rate (in m3 m–1 a–1), Vm is volumeof sediment landing outside the pits and which form mounds(in m3), d is distance (in meters) that sediment is ‘transported’along the ground (the net ground-parallel distance travel afterthe sediment has been upheaved and falls to the ground), Ap

is area of survey plot (in m2) and t is the estimated number ofyears that the root plates have existed.

Volume is calculated as the product of original root platevolume, the percentage that had fallen off the root plate, andthe percentage falling outside the pit (this value must becalculated for uphill topples and is assumed to be 100% for

downhill topples; see explanation later). Sediment transportwas calculated for each survey plot, providing a transport ratefor three different gradients. This analysis is completed forroot plates of Age Classes 1–3, and thus t is 32 years.

Sediment transport distance was calculated using geometricmodels based on field measurements, similar to the approachof Gabet et al. (2003), but with two major differences. First, thecurrent study does not consider the entire root plate volumein the transport calculations, but excludes the volume of rootplate sediment that will fall back into the pit. Second, this studyexamines uphill and downhill sediment transport separately,rather than calculating a single net transport value. The modelinvolves two components contributing to transport distance:(i) slope-parallel transport distance during upheaval; (ii) slope-parallel transport distance during root plate disintegration.Two assumptions are required for this model: (i) root platesare assumed to be circular, with the dimension of the rootplate perpendicular to the ground representing diameter; and(ii) the root plate comes to rest with the center of mass at theedge of the pit (Figure 5A).

The ground-parallel transport distance for downhill topplesis now considered. The net downslope distance for upheavalassociated with downhill topples is the distance between two‘contour lines’ situated across the slope, one which crossesthrough the center of the pit and the other crossing throughthe center of the root plate mass. This plan view distance isconverted into true distance by making an adjustment forslope gradient. During the disintegration phase, all sedimentfor downhill topples is assumed to fall outside the pit, withsimple geometric calculations used to calculate the netdownslope transport distance for this stage of the root throwprocess. The net ground-parallel transport distance used incalculations is shown in Figure 5(B).

Similar geometrical considerations are applied to the modelfor uphill topples, but it cannot be assumed that all sedimentfalls outside the pit. In addition, there is now both an uphilland downhill component to the transport process, as theupheaval moves sediment uphill, while disintegration movessediment downhill; both must be accounted for to determineif there is net uphill or downhill transport. Once again, geometricmodels are applied to determine the amount of sedimentassociated with the root plate that will fall outside the pit. Aline in the direction of steepest gradient is imposed tangentiallyto the rim of the pit. The intersection of this line with the rootplate half-ellipsoid (sitting at some angle above the pit)determines the proportion of sediment that will ultimatelyfall outside the pit; only this sediment is used in transportcalculations (sediment landing in the pit is not considered tobe part of the transport process).

In Survey Plot 1, where gradient was too low to accuratelydetermine contour line direction, only the disintegrationtransport distance was used, resulting in understated transportdistances. The impact of this is expected to be small, as verylittle sediment is transported by root throw (i.e. very littlesediment falls outside the originating pits) on low gradientslopes.

Table VIII. Sediment disturbance values

Surveyplot


Area of plot (ha)

Volume uprooted over 32 years (m3)

Annual volume disturbed (m3 m–2 a–1)

Average depth(mm a–1)

1 3 2·5 10·01 1·25 × 10–5 0·0132 15 4·1 21·45 1·63 × 10–5 0·0163 28 3·2 42·49 4·15 × 10–5 0·041

Note: Sediment disturbance includes estimates of all sediment that was originally uprooted, although some of it may have since disintegrated andformed part of a mound.

Table VII. Total pit area disturbed and annual rate of pit formation

Surveyplot


Total pit area(m2 ha–1)

Annual totalpit area (%)

1 3 28·7 0·0032 15 36·0 0·0043 28 59·6 0·006

qV dA ts

m

p

=Σ( )



Uphill and downhill sediment transport rates during thepre-fire period are shown for root plates categorized in AgeClasses 2 and 3 (Figure 6A); sediment is considered as havingbeen transported once it falls to the ground from the rootplate. Sediment transport rates are also calculated for the twopost-fire years (Figure 6B); these data include trees thattoppled during both the post-fire and pre-fire periods (AgeClasses 1–3), but for which sediment actually fell to theground in the post-fire period. We first discuss the downslopetransport rates. Negligible sediment transport occurred on thelowest gradient plot for the pre-fire period, as sediment wasusually returned directly to the pit. Moreover, transport rates

remained negligible for the mid-gradient plot, and it is onlyfor our upper-gradient plot, having a slope of 28°, thatnotable increases in sediment transport rates were observed.There appears to be a non-linear form to the plot; however,with so few data points, broad conclusions should not bemade. The post-fire results show the same pattern in the data,with the major difference being an approximately one-order-of-magnitude increase in sediment transport for thepost-fire scenario versus the pre-fire scenario. Upslope transportrates for both periods show negligible values on the low-gradientfield plot, with somewhat higher values for the steeper-gradient field plots.

Figure 5. (A) Circular pit and root plate, with center of mass of root plate situated at edge of pit. (B) In the model, the first stage of sedimenttransport involves the movement of sediment from what becomes the pit to the new location of the root plate after tree topple. The second stageof transport involves the vertical disintegration of sediment to the ground, with the distance for this stage defined as the distance between the rootplate centroid to the location where vertically falling sediment reaches the land surface (shown by the bold arrow). The double-headed arrowshows the ground-parallel net transport distance associated with root throw transport.

Figure 6. Downslope and upslope sediment transport plots. (A) Transport results for the pre-fire period. (B) Transport results for the two post-fireyears. Note scale differences on the two graphs.



Model Outline

Model overview

While the field results provide an indication of sedimentdisturbance and transport rates due to root throw, the temporaldynamics of root throw-driven transport is directly related tothe population dynamics of forests. To obtain greater insightsinto these temporal dynamics, we develop a combined treepopulation dynamics/sediment transport model that is calibratedfor the central Canadian Rockies. In simple terms, this modelgerminates and grows trees, kills trees, and has these trees fallto the ground. A portion of these fallen trees generates rootthrow events. Fire events occur as a disturbance that kills alltrees and results in a new forest. Thus, the model cyclesthrough forest generations with life spans determined by thevariability of fire event recurrence. Calculation of the sedimenttransport by root throw is organized as follows: first, betweenwildfires the population dynamics of the trees is modeledusing the algorithms for recruitment, mortality, and growth indiameter. Next the timing of mortality of all the trees in thepopulation from crown fire is determined based on informa-tion about fire return intervals. The tree falling (topple) rateand chance of uprooting versus breakage of the dead trees,either from fire or from inter-fire mortality, are determined.Finally, the amounts of sediment associated with root platesof toppled trees are determined and the transport rates arecalculated. A list of model parameters is given in Table IX.

Fire frequency

The lifespan of forest trees in the study region is largely deter-mined by fire return intervals and type of fire. Mean firereturn interval in a region is estimated as the expected numberof years between fires. The Weibull probability density function

for fire intervals expresses the probability of having fires withinter-fire intervals of length t (Johnson and Van Wagner, 1985):

(4)

where t is time interval between two fires (in years), α is ascale parameter (expected fire return interval in years) and γ isa shape parameter (dimensionless). For this modeling exercise,the shape parameter γ is set to one, resulting in a negativeexponential distribution, which studies in the area have shownto provide a good fit to the data (Masters, 1990; Johnson andLarsen, 1991; Reed et al., 1998). A value of 110 years ischosen for the variable α. Random numbers are selected fromthe Weibull distribution, which represent inter-fire time intervalsfor the model run.

Tree population dynamics

In the subalpine forests of lodgepole pine (Pinus contortaLoudon var. latifolia Engelm.) and Engelmann Spruce (Piceaengelmannii Parry ex. Engelm.) found in the region understudy (Johnson and Fryer, 1989; Johnson et al., 1994; Johnsonet al., 2003), two groups of trees contribute to tree topplingand sediment transport. The first group consists of trees eitherkilled by the last fire or dead standing before the last fire, andthe other group consists of trees that are recruited after thelast fire. The first group represents a cross-section of the livetrees and standing dead trees at the time of the last fire,showing a range of diameters. The second group considersthe recruitment and mortality of trees in two kinds of cohort.A cohort is a group of trees recruited at the same time andfollowing a similar mortality schedule throughout their lives.The fire cohort is defined as those trees recruited in the five-to 10-year period after the fire in which the canopy was killed(Johnson et al., 2003) and the forest floor was removed bysmoldering combustion (Charron and Greene, 2002; Miyanishiand Johnson, 2002). The understory cohorts are those that beginto grow under the fire cohort’s canopy and on a forest flooroccupied by groundcover.

Both the recruitment (Figure 7A) and mortality rates (Figure 7B)used in our simulations are from Johnson and Fryer (1989)and Johnson et al. (2003). The duration of the fire cohort isthe period of time for canopy trees to become established,and in our model its duration is assigned a value of 10 years.Afterwards, all new recruits are considered to be part of theunderstory cohort and, as seen in our recruitment curve, itgenerally has a lower recruitment rate. Mortality rates arespecified separately for each cohort (Figure 7B). The understorycohort has a higher mortality rate than the fire cohort. Mortalityrates are related to age of trees, and within one time intervaldifferent mortality rates are applied to trees of different agesand different cohorts.

Root throw events

Two groups of trees contribute to root throw and transport:(i) trees either killed by fire or dead standing before fire; and(ii) trees recruited after fire with different mortality rates forthe two cohorts. Field data (Johnson, 1986, unpublished data)show that topples occurring in the first several years after fireof fire-killed trees or dead standing trees at the time of the fireresult from having their root support weakened by removal of

Table IX. Input parameters for model simulations

Number of years of simulation 1000 yearsPlot area 100 m2

Hillslope gradient 20°Average fire return interval (scale parameter) (Equation 4)

110 years

Shape parameter for fire (Equation 4) 1·0Duration of the fire cohorta 10 yearsBetween-fire falling rates (F in Equation 5)b

Fire cohort 0·058c Understory cohort 0·058

Post-fire falling rates for all fire killed trees 0·084d

Fraction uprooted versus broken boles 0·8Parameters to assign dbh distributions for trees of certain ageScale parameter Mean dbh values

(Figure 8) Shape parameter 1·8dbh threshold for uprooting mass 13 cm

a The user input for span of fire cohort will determine when the canopyis formed and new recruits are going into the understory cohort ratherthan the fire cohort.b Fire ‘cohort’ is defined as those trees that germinate after fire andform the canopy. Understory ‘cohort’ is defined as those trees thatgerminate after the canopy is formed.c The value of 0·058 is based on data for Johnson and Greene (1991)and Johnson (unpublished data) for the Kananaskis Valley.d The value of 0·084 is based on Lyon (1977). Fire killed trees maygenerate root throw until next fire.

f tt

et

( ) = ⎛⎝⎜

⎞⎠⎟

− −⎛⎝⎜

⎞⎠⎟γ

α α

γα

γ1



organic matter around their bases (Miyanishi and Johnson,2002) and burning of roots during the fire. Subsequent post-fire tree topple is due to loss of root strength (Martin andJohnson, 2004–2008, unpublished data). Dead trees will fallby uprooting or breakage.

Toppling of standing dead trees occurs according to anexponential model (e.g. Lyon, 1977, who used this model forpost-fire topples), whereby a constant fraction of the remainingstanding dead boles falls in any given year. The fraction ofdead trees that topple in a given time interval is obtainedfrom Equation 5:

FT = 1 − exp(−F ∗ dt) (5)

where F is a parameterization based on field studies of fractiondead standing trees that topple and dt is the time interval (inyears) used in our model runs. In our model, parameterizationwas based on falling rates taken from studies by Johnson andGreene (1991), Johnson (1986, unpublished data) and Lyon(1977) (see Table IX for further details). The calculated fractionis applied to the remaining standing dead boles for each timeinterval to determine the number of trees that topple in thatperiod.

Empirical surveys show that in our study area about 80% oftrees uproot as opposed to breaking, and thus constitute rootthrow events (Johnson, unpublished data); this value is incor-porated into the model to obtain the actual number of rootthrow events after the number of tree topples has beencalculated. Tree fall directions are random in the model runsreported herein.

Root plate volume, necessary to calculate sediment distur-bance and transport, is dependent on tree size, which in turnis related to tree age when the root throw event occurred(recall that a tree topple is only considered a root throw eventwhen sediment upheaval is involved). The distribution rangefor dbh is influenced by tree density in the forest (Harper,1977); higher density forests have smaller dbh values. Treedensity is dependent on factors such as soil conditions, treespecies, topography, climate, and probably other factors. Thedbh distribution in the present model is based on a density of2000 trees per hectare as found in our field plots [refer to ourfield data and also Smithers (1961)], and requires adjustmentif applying the model to different locations. For an event to beincluded in our sediment disturbance and transport calcula-tions, the relevant tree must have a diameter >13 cm. Thisvalue provides an important condition that must be met inthe model for notable sediment upheaval to occur when atree topples (Johnson, unpublished data).

To assign dbh in the model, mean tree diameter is first deter-mined for a tree having a certain age in the model (Figure 8).To ensure a more realistic distribution of diameters for trees ofa certain age, a two-parameter Weibull distribution is used toassign actual diameters in the model, with the scale parameterdefined as the mean diameter for trees of that age and theshape parameter being assigned a value of 1·8 (tree diameterdata are based on unpublished data of Johnson for theKananaskis Valley, Alberta).

Root plate volumes

Two pieces of data provide the basis to assign root platecharacteristics for each root throw event in the model: theage of the tree when it died and its dbh (see earlier), andthe year of fall. Root plate volume is based on dbh using theregression equation obtained from field data for Hawk Creek(Gallaway, 2006) (see Equation 2).

Sediment transport

In the model, disintegration distributes the transfer of sedimentfrom the root plate to the ground over 100 years and iscalculated based on an empirical best fit to our field data(Gallaway, 2006):

Figure 7. (A) Recruitment curve used in model runs. Trees recruited within the first 10 years after a fire are part of fire cohort and any treesrecruited thereafter are part of understory cohort. (B) Mortality schedule for understory and fire cohorts. Based on Johnson and Fryer (1989) andJohnson et al. (2003).

Figure 8. Mean diameters (dbh) for trees of a particular age. Toensure a realistic distribution of diameters for each age of tree in themodel, a Weibull distribution is used in conjunction with the meandiameter to assign a range of diameters. Based on unpublished databy Johnson for the Kananaskis Valley, Alberta.



PV = −0·1029 + 46·19e−0·719t + 56·35e−0·02102t (6)

where PV is the percent volume remaining and t is time inyears. The exponential form of the equation suggests that thedisintegration rate may stay constant as the volume decreasesover time. The volume falling off a root plate in a time intervalis the difference in the percent remaining as calculated fortwo points in time. The volume of interest for transport is thevolume of sediment that falls outside the originating pit; thisis dependent on the fall angle of the tree (i.e. if a tree fallsdirectly upslope, it is assumed all sediment will return to thepit). Conversely, if a tree falls directly downslope, all sedimentwill fall outside the pit. The proportion of sediment fallingoutside the pit is calculated as:

PS = FA/180 (7)

where PS is the proportion of sediment falling outside the pitand FA is the falling angle of the tree. Transport distance isthe net slope-parallel distance after upheaval and disintegration.Direction of this net distance may be: (i) downslope; (ii) upslope,if upslope upheaval distance exceeds downslope disintegrationdistance; or (iii) back into the pit. Ground-parallel transportdistance for sediment falling off a root plate is based on aseries of geometric calculations similar to those described forthe field data, and which requires a knowledge of root plateheight (the pit is assumed to be circular with a diameterhaving this same value). The height dimension needed tocalculate transport distance is obtained from the followingequations:

wRP = 0·65 + 4·65dbh R2 = 0·62 (8A)

hRP = 29·24 + 0·54wRP R2 = 0·62 (8B)

where dbh is diameter at breast height (in cm), wRP is rootplate width (in cm), and hRP is root plate height (in cm).

Transport for a particular time interval is calculated for thesum of sediment volume falling off all existing root plates inthat time interval in conjunction with its transport distance.

(9)

where qs is sediment transport rate (in m3 m–1 a–1), VRP isvolume of sediment disturbed for a root plate during aparticular event (in m3), distRP is travel distance associatedwith that event (in meters), and At is the area of the plot beingmodeled (in m2).

Model Results

Results for an example millennial-scale model run are presentedto highlight trends in temporal variations in tree populationsand associated tree mortality, tree toppling, root throw, sedimentupheaval, and disintegration. Values of the parameters usedin the model run are given in Table IX.

Fire-killed or standing dead trees at time of fire

Fires are the critical disturbance process in our modeled forest,as fire events control the temporal dynamics of tree mortality,tree toppling, and root-throw sediment processes. Random

fire distributions (see Equation 4) for this model run show arange of fire return intervals (time period between successivefires), with intervals as short as 15 years and up to 300 years(Table X).

Figure 9 illustrates examples of tree age distributions at thetime of fire for several fires with different lengths of time sincethe previous fire. The numbers and sizes of trees at the timeof fire are a significant determinant of post-fire sedimentdisturbance and transport. The duration of time since theprevious fire is the time period available for tree growth,and the trees are then subjected to fire-driven mortality andsubsequent toppling. The number of new root throw eventsrepresents trees that not only have died and toppled, but thatalso have upheaved sediment (Figure 10). Important to note isthat trees do not immediately topple after they die, but ratherthey follow an exponential toppling rate based on time sincedeath. Therefore, we expect a time lag between tree mortality,toppling, and associated sediment disturbance and sedimenttransport processes.

When the time interval since the previous fire is short, verylarge numbers of trees will be standing at the time of fire, buttheir dbh values will be small due to their young age andthe dbh of most trees is below the threshold of 13 cm forsediment upheaval (refer to Figure 9A). Hence, the mortalityand eventual toppling of these fire-killed (or dead standing)trees involve very small amounts (or no) sediment and theyare not included as root throw events. For example, the fireat Year 805 represents a disturbance having a short value of time-since-previous-fire, and no root throw events for fire-killedtrees occur immediately following the fire (refer to Figure 10).

When the fire return interval exceeds the time needed fortrees to reach the critical dbh, the fire-killed trees contributeto root throw events and to sediment disturbance (refer toFigure 9E). An example of this is the fire occurring at Year580, which shows a notable post-fire increase in root throwevents (refer to Figure 10). In such cases, both the number oftrees above the critical dbh and their sizes are important indetermining the total amounts of sediment upheaved.

Once the time interval since the previous fire exceedsapproximately 125 years, the time when tree mortality of firecohorts decreases to very low levels in our model simulation,the actual number of trees in the plot remains approximatelyconstant (refer to Figures 9D and 9E). However, these treescontinue to grow in size, and the amount of sedimentinvolved in the toppling of the trees will likewise increase.

Sediment disturbance is directly tied to the number of newroot throw events and, to a large extent, should follow thepatterns of new root throw events for fire-killed or deadstanding trees at the time of fire (Figure 11), as well aspatterns of mortality and toppling as the forest establishes

qV

Ai

n

s

RP RP

t

*dist= =

∑ ( )1

Table X. Times of fire in example model runa

Time of fires (years since start of model run)

50 130 235 280 580 790 805 870 920

a Model assumes a new forest begins to grow at time 0. No fire-killedtrees exist at this time as no previous forest exists.



itself in between fire events. Sediment is only considered ashaving been ‘transported’ once it disintegrates, with disintegra-tion not being instantaneous but rather a process that isextended in time. Therefore, while we expect a broadly similar

pattern for sediment transport as for sediment disturbance,the disintegration process results in a slight dampening andextending out of the temporal patterns that are found forsediment disturbance (Figure 12). Annual rates of sediment

Figure 9. Tree age distributions at time of fire. The distributions are shown for fires at the following times. (A) 805 years (previous fire interval of15 years); (B) 870 years (previous fire interval 65 years); (C) 235 years (previous fire interval of 105 years); (D) 790 years (previous fire interval of210 years); (e) 580 years (previous fire interval of 300 years).

Figure 10. New root throw events for model run. Counts are for five-year bins. Years of fire events are shown by triangular symbols.

Figure 11. Volume of sediment disturbance for model run. Years of fire events are shown by triangular symbols. Results are grouped into five-year bins.



transport show marked variability that changes in responseto the tree population dynamics. The frequency distributionof annual transport rates for our millennial-scale model runis shown in Table XI. The mean value for all annual transportrates is 0·0012 m3 m–1 a–1, with a standard deviation of0·0047 m3 m–1 a–1. Values range from 0 m3 m–1 a–1 to amaximum of 0·059 m3 m–1 a–1.

Understory and fire cohorts

After fire occurrence, fire-killed or dead standing trees beginto topple. During this same period, new recruits begin topopulate the stand in large numbers, and mortality of thesetrees also occurs in relatively large numbers. After about 50to 60 years, a second pulse of root throw events begins tooccur for several reasons. Firstly, the mortality rate of trees inthe fire cohort increases significantly after about 60 years (seeFigure 7B). Furthermore, trees that have reached this age maybegin to have dbh values that are >13 cm (critical dbh forsediment upheaval). Once dbh values reach 13 cm (whichoccurs on average after 80 years, although the random com-ponent for tree dbh in the model allows some trees to reachthis size several decades sooner), they begin to upheave notableamounts of sediment as they topple. An example can beobserved in Figures 10 and 11, with notable sedimentdisturbance beginning at about 50 years and increasing forthe next several decades after the fire event at Year 805. Priorto these conditions being met, trees in the two cohorts thatare subject to mortality have little impact on sediment distur-bance or transport as they are too small to upheave notableamounts of sediment. Therefore, in addition to a pulse ofsediment upheaval due to fire-killed or dead standing trees inthe immediate post-fire years, at about 50 years onwards one

may begin to observe a second pulse of sediment upheavaland associated transport (Figures 11 and 12), with this pulseexpected to last several decades, after which time treemortality decreases to background levels.

Discussion and Conclusions

Sediment transport rates by root throw have been estimated insome early studies on this topic (Denny and Goodlett, 1956;Reid, 1981; Mills, 1984). However, the connection betweenroot throw and forest population dynamics, with the latterdriving the former by the process of tree toppling and rootupheaval, was not fully explored in these studies. Gabet et al.(2003) expanded on earlier work by developing a model topredict medium-term rates of root throw transport as a functionof hillslope gradient. Their model accounts for tree topple rates,with the assumption that uprooting rates in the forest aretemporally constant at a rate of four trees per hectare perannum. Their model utilizes results of previous field studies(e.g. Norman et al., 1995) to obtain values for root platedimensions, and uses this information in conjunction with aseries of geometrical relationships pertaining to tree fall toestimate transport rates. Some additional assumptions weremade in their model, such as values of dbh do not changeover time as trees age, and therefore root plate volumes alsodo not change with time. The results of Gabet et al. (2003)provide first-order approximations of medium-term sedimenttransport rates due to root throw and mark notable progress inour understanding of this process.

Without an explicit connection to forest disturbance andtree population dynamics, the temporal dynamics of the rootthrow process cannot be fully realized. To our knowledge ourstudy represents the first attempt to explicitly combine amodel of root throw with a detailed rendering of tree popula-tion dynamics to explain how ecological forcing drives thetemporal aspect of sediment transport by root throw. Becausethere were no existing regional field data/observations of rootthrow on which to base model calibrations and help makeinferences for our work in the Canadian Rockies, it wasnecessary to include a field component in our study.

Tree topple rates may increase in the immediate post-fireyears as root support is weakened by the removal of organicmatter around the tree and due to the burning of roots duringthe fire. The marked post-fire increase in toppling for thesteep gradient class relative to the less steep gradient classesfound in our field data may occur because the structural stabilityof trees is lower on steeper slopes (Quine and Gardiner, 2007).Our field data identified the relation between dbh, whichincreases as trees age, and root plate volumes in our region;such data is critical to integrate sediment transport with treestand ages in our model. The field data demonstrate a signi-ficant dependency between gradient and root plate volume and

Table XI. Frequency table of sediment transport rates for model runof duration 103 years

Annual sediment transportrate (m3 m–1 a–1) Frequency

0–0·0005 1280·0005–0·0010 150·0010–0·0015 180·0015–0·0020 90·0020–0·0025 90·0025–0·0030 50·0030–0·0035 70·0035–0·0040 20·0040–0·0045 20·0045–0·0050 20·0050–0·0055 1>0·0055 3

Figure 12. Annual sediment transport rates for model run. Years of fire events are shown by triangular symbols. Results are grouped into five-year bins.



all individual dimensions (width, length, depth). An increasein the ratio of length to width was found at steeper gradientsrelative to less steep slopes (significance level of p < 0·05). Itmay be the case that, as hillslopes become steeper, tree rootsspread perpendicular to contour lines to impart greaterstability to the tree (Coutts et al., 1999). Further investigationof this idea is suggested, as such information would allow forimproved understanding of the resistance offered by trees tomortality-induced toppling, wind events or landsliding.

The pit-mound features formed by tree uprooting and rootplate disintegration result in a unique microtopography thatcontributes small-scale roughness elements to the landscape.These roughness elements may affect depression storage andponding of water during large storm events, which may affectthe timing of hydrological responses in the watershed (Martinet al., 2008). The mound provides an exposed, unconsolidatedsupply of sediment for rainsplash or other water-driven processesprior to colonization of significant vegetation cover on thefeature, while soil creep may act to diffuse the mound bothprior to and after vegetation recolonization. It is recommendedthat future studies explore sediment transport processes operatingon pit-mound features to better understand their longevityand contribution to microtopography in forests. Osterkampet al. (2006) found an average disturbance rate of the soil byroot throw of 0·0095% of the surface per year, which iswithin an order of magnitude of our values. Our data suggestthat a complete turnover of the soil surface has not likelyoccurred in our study area since post-Pleistocene reforestationwhich began <10 000 years ago. Other studies, summarizedin Schaetzl et al. (1989), report disturbance cycles from 220to 3174 years in eastern North American forests.

To allow for comparison with annual depths of sedimentdisturbance due to root throw for other processes reported inthe literature, such as soil creep or landsliding, the volumeof sediment disturbed per meters squared of land surface isconverted into an annual depth of disturbance by dividing by32 years, the approximate time period of the root plate upheavalfor Age Classes 1–3. Annual disturbance values are of the order1·0 × 10–2 mm a–1. This result is only one-order of magnitudelower than typical annual disturbance depths reported forshallow landsliding in coastal British Columbia, with a valueof about 1·0 × 10–1 mm a–1 (Martin et al., 2002). This suggeststhat upheaval of sediment due to tree topple is a notable factoroperating on forested hillslopes. When considering sedimenttransport (incorporating both upheaval and disintegration) anotable percentage of disturbed sediment is often returned tothe pit, particularly for our lowest gradient plots, and in suchsituations does not contribute to net transport of sediment.There is approximately a one-order of magnitude increase insediment transport for the post-fire versus the pre-fire rates forall gradient classes. It is only for our steepest-gradient plot thatnotable values of sediment transport were observed. Althoughconsiderable volumes of sediment are involved in root throwevents (resulting in notable values of sediment disturbance),the associated transport distances are relatively small (5 cm to150 cm), leading to relatively low transport rates. Given thelimited time window of our field study, the millennial-scaletransport estimates derived in our model, which take intoconsideration the temporal dynamics of sediment transport,may be preferred.

In general, vegetation properties are treated very simply inmany geomorphic studies. The complex effects of vegetationon sediment transport are often encapsulated in variousparameters within equations, rather than being explicitlyconsidered. However, within our model, the details of treepopulation dynamics (recruitment and mortality) play a verydirect role in determining the timing and pulsing of root

throw sediment transport. In particular, two notable pulsesof transport due to root throw are evident in our results: (i) iftrees are large enough to have reached a critical dbh at timeof fire, then a pulse of sediment occurs in post-fire years,which decreases exponentially with time since fire; and(ii) once new recruits have reached a critical dbh and withcompetition mortality (thinning), then a second pulse of rootthrow begins at about 50 to 60 years after the previous fire. Ifthe period of time since the previous fire is not sufficient forenough standing trees to have reached a critical dbh, thentrees may topple but they will not upheave notable amountsof sediment. It is only when the time since previous fireexceeds the time needed for critical dbh to be reached thatnotable sediment disturbance and transport begins to occur.After about a century the number of trees does not decreasevery much and dbh continues to grow, increasing root platevolumes for the trees, which eventually leads to increasedsediment disturbance and transport.

Transport rates for our millennial-scale model run are nowcompared to published results in the literature. The model ofGabet et al. (2003) estimates an annual transport rate due toroot throw of 1·6 × 10–3 m3 m–1 a–1 for a 20° slope. Roering et al.(2002) estimated millennial-scale transport rates for biogenicprocesses (e.g. root growth, root throw) based on vertical profilesof tephra concentration and topographic derivatives. Theresulting K value (i.e. the diffusion coefficient, which representsthe transport rate at unit gradient or 45°) is 1·2 × 10–2 m3 m–1 a–1.Since the form of this transport equation is linear, then thetransport rate for a slope gradient of 20° (the slope gradientused in our model runs) can be inferred and has a value of4·4 × 10–3 m3 m–1 a–1. Walther et al. (submitted for publication)estimated the diffusion coefficient for soil movement by rootgrowth, bioturbation and tree throw based on the distributionof tephra grains in soils found in southeast Washington State.They obtained a diffusivity value of 4·8 × 10–3 m3 m–1 a–1,which translates into a transport rate of 1·7 × 10–3 m3 m–1 a–1

for a slope gradient of 20°. Results for our millennial-scale model run provide a mean annual transport rate of1·2 × 10–3 m3 m–1 a–1, which compares favorably with thelonger term values obtained for the above-mentioned studies(all values have the same order of magnitude). For furthercomparison, Martin and Church (1997) reported an averagelinear diffusion coefficient based on a large number of soilcreep rates reported in the literature of 2 × 10–4 m3 m–1 a–1

(7·3 × 10–5 m3 m–1 a–1 for a 20° slope), and reported adiffusion coefficient for debris slides of 1 × 10–1 m3 m–1 a–1

(3·6 × 10–2 m3 m–1 a–1 for a 20° slope); our transport rates for

root throw are between these two values.This study provides an example of how direct the links

may be between ecology and geomorphology. In our fieldand modeling studies, tree age and size as determined by theforest population dynamics was shown to be a key factoraffecting the timing of root throw and the amount of sedimentassociated with these events. It is suggested that futuregeomorphological studies attempt to strike a balance betweenincorporating vegetation in a more realistic manner than hasoften been the case in past geomorphic studies, while at thesame time keeping our understanding of the joint processestractable.

Acknowledgements—We acknowledge the financial support of theBiogeoscience Institute and NSERC Discovery Grants to EAJ andYEM. We also thank Kootenay National Park for the support providedfor the field component of this project. Numerous field assistantsprovided excellent assistance in the field and contributed to thesuccess of this project. The manuscript benefited from the perceptivecomments of two anonymous reviewers.



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